A crystal is a solid material made up of atoms, molecules, or ions arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth occurs by the addition of new atoms, molecules, or ions, into the characteristic arrangement of a crystalline lattice. The type(s) of atoms, molecules, or ions that make up the crystal can form one or more typical crystical lattices. Crystal growth typically follows an initial stage of either homogeneous or heterogeneous (surface catalyzed) nucleation, unless a “seed,” on which crystal growth can begin, is already present.
Monocrystalline silicon or single-crystal silicon is the most common material of the electronic industry. It is made up of silicon in which the crystal lattice of the entire solid is continuous and unbroken (with no grain boundaries) to its edges. It can be made of pure silicon or doped with small quantities of other elements to change its semiconducting properties. Layered silicon-insulator-silicon substrates have also been used in place of silicon substrates in semiconductor manufacturing, especially microelectronics, to improve performance.
With the increased demand in the performance and complexity of microelectronic and optoelectronic devices, there is an increasing need to integrate dissimilar devices and materials onto the same chip for increased performance and reduced cost. At present, the silicon microelectronics industry has reached a state of maturity, in that 12″ to 16″ wafers are readily available, and foundry services are much more affordable. Other electronic and photonic materials (GaAs, InP, GaN, Ge, InSb, and others) can benefit tremendously from tapping into existing momentum and infrastructures that have been developed over the past five decades.
Epitaxy of semiconductor layers has enabled contemporary optoelectronic and microelectronic industry. A fundamental prerequisite in epitaxy is the availability of crystalline substrates having a reasonable match in crystallographic structure and atomic registry. The ability to prepare single crystalline layers on amorphous or polycrystalline substrates is a tantalizing yet conceptually daunting quest: on a surface with no long-range atomic ordering, it is challenging to proceed with nucleation, incorporation, and growth to produce a macroscopic-scale crystal.
Two of the common methods in attempting the heterogeneous integration (HI) are (1) heteroepitaxial growth, and (2) layer transfer through wafer bonding. The former involves material research in overcoming mismatches in lattice parameters, crystallographic configurations, and coefficients of thermal expansion among the materials to be integrated together. Notable examples include heteroepitaxy of GaAs on silicon, Ge on silicon, and InAs on GaAs. Method (1) is generally ineffective in supporting the integration of a crystalline layer to a polycrystalline or amorphous substrate. The wafer bonding technique of (2) requires selective substrate removal or layer lift-off techniques together with sophisticated alignment processes. Wafer-bonding has much more flexibility than heteroepitaxy described in (1) but at the expenses of a much higher cost and lower yield. It is therefore only used in highly specialized applications with a very high profit margin.
One approach to preparing crystalline layers on amorphous or polycrystalline substrates has employed artificial epitaxy, or graphoepitaxy1, which resorts to nanoscale surface patterning, typically through electron-beam lithography, to create geometric boundary conditions on the length scale of nuclei so their orientations can be influenced, if not dictated, by the shape of the nanomolds2,3. With all the advances in growth and lithography techniques since then, however, graphoepitaxy has not produced the level of control required by modern device applications.
Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in bright light-emitting diodes. The compound has a Wurtzite crystal structure. Its wide band gap gives it special properties for applications in optoelectronic, high-power and high-frequency devices. Choi et al., “Nearly single-crystalline GaN light-emitting diodes on amorphous glass substrates,” Nature Photonics 5:763 (2011) describes the preparation of nearly single-crystalline GaN-based light emitting diodes on amorphous glass substrates. A “pre-orienting layer” of thin-film titanium is used to affect the growth direction of a subsequent GaN nucleation layer. Titanium has the same hexagonal crystal lattice as wurzite GaN. Spatial confinement of nucleation sites was achieved by placing a hole-patterned SiO2 layer on the LT-GaN nucleation layer. GaN arrays were formed on these nucleation sites during high-temperature GaN growth. In the initial stage of high temperature GaN growth (HT-GaN), a number of crystal islands began to grow and compete with one another. These GaN islands have aligned out-of-plane, but there are also random in-plane orientations. The randomness of in-plane orientations inhibits or retards the coalescence of the islands as HT-GaN growth progresses. The nucleation sites can be further confined by reducing the dimensions of the hole, so each hole will only have one dominant island. Using this mechanism, nearly single-crystalline GaN pyramid arrays can be fabricated on amorphous glass substrates through the formation of spatially confined nucleation sites with preferred c-orientation. The GaN pyramid arrays were incorporated into LEDs.
US Patent Application Publication No. 20120025195 relates to confined lateral growth of crystalline material. A lateral growth channel is provided between upper and lower growth confinement layers, and is characterized by a height that is defined by the vertical separation between the upper and lower growth confinement layers. A growth seed is disposed at a site in the lateral growth channel for initiating crystalline material growth in the channel. The seed material can be monocrystalline, polycrystalline or amorphous, though an amorphous morphology is preferred. A growth channel outlet is included for providing formed crystalline material from the growth channel. With this growth confinement structure, crystalline material can be grown from the growth seed to the lateral growth channel outlet. The structure is used to produce single crystal germanium. While the publication states the structures can be applied to any material for which crystalline growth is desired, such as other II-VI as well as III-V polycrystalline and monocrystalline materials can be produced, no actual examples besides germanium are provided, nor are growth conditions provided for other materials. The publication does not refer to the use of a textured thin film, nor does it disclose growing a crystalline material in the growth channel along a direction that is substantially perpendicular to the preferential crystallographic axis of the textured thin film.
McComber et al., “Single-Crystal Germanium Growth on Amorphous Silicon,” Adv. Funct. Mater. 22:1049-1057 (2012), presents a method that employs the selective growth of germanium on amorphous silicon by ultra-high vacuum chemical vapor deposition (UHVCVD) at low temperatures (T<450° C.). It reports much of the same information as US Patent Application Publication No. 20120025195, including growth confinement structures where Ge selectively grows from areas of exposed amorphous silicon. The paper states that its work concerns the deposition of polycrystalline germanium (Ge) and concentrates on Ge and its diamond cubic lattice.
US Patent Application Publication No. 20130029472 relates to a gallium nitride (GaN) bonded substrate and a method of manufacturing a GaN bonded substrate in which a polycrystalline nitride-based substrate is used. The method includes loading a single crystalline GaN substrate and a polycrystalline nitride substrate into a bonder; raising the temperature in the bonder; bonding the single crystalline GaN substrate and the polycrystalline nitride substrate together by pressing the single crystalline GaN substrate and the polycrystalline nitride substrate against each other after the step of raising the temperature; and cooling the resultant bonded substrate. Thus, the single crystalline GaN substrate and the polycrystalline nitride substrate are grown separately and bonded together.
Liv et al., “High-quality single-crystal Ge on insulator by liquid-phase epitaxy,” Applied Physics 84(14): 2563 (2004) discusses a single-crystal germanium layer grown on a nitride insulator. A nitride layer was formed on a crystalline silicon substrate with a seeding window, then Ge was sputtered onto the nitride and the silicone through the seeding window. A low-temperature oxide was deposited over the Ge, forming a micro crucible. The silicon substrate was heated to 940° C., causing the Ge to melt. Upon cooling, a single-crystal Ge grew from the Ge/Si interface in the seeding window, extending over the nitride.
The growth of device-quality single crystal GaN on an amorphous or polycrystalline substrate remains an unmet challenge, one which could provide tremendous benefits in the fields of optoelectronics, microelectronics and others.